NAD(P)+-NAD(P)H Models. 55. Transition Metal Catalyzed Reduction of Organic Halides: High Selectivity for Reductive Dehalogenation.

Article abstract of DOI:10.1021/jo00218a008

Various types of aryl, benzyl, vinyl, phenacyl, and allyl halides were subjected with N-benzyl-1,4-dihydronicotinamide (BNAH), in the presence of a catalytic amount of chlorotris(triphenylphosphine)rhodium(I) or palladium(II)acetate.The carbon-halogen bonds in these compounds were selectively reduced to the carbon-hydrogen bonds in moderate to excellent yield.This new kind of reduction system has synthetic advantages over many other procedures for reductive dehalogenation: the conditions are mild, nitro, carbonyl, hydroxyl, amino, alkenyl, and ester groups are inert under these conditions, and the halides that can be used are varied.The order of reactivity in a series of organic halides was Cl < Br < I.Substituent effects in the aryl iodides showed that the stronger the electron-withdrawing ability of the substituent, the more reactive the substrate.These results and other evidence suggest that the reaction involves oxidative addition of halides to the transition metal.The reduction of vinyl bromide gave predominantly the thermodinamically more stable isomer, after cis-trans interconversion.In addition, this reduction was accompanied by contamination by deuterium from the deuterated solvent.These facts reveal the intermediacy of vinyl free radicals in the course of the reaction.The free-radical species is probably generated through an electron transfer from BNAH.